An airbag provided to ensure the safety of an occupant in an automobile at the time of collision is usually mounted in a small space such as a steering wheel or an instrument panel in the form of a module in which the airbag is stored in combination with an inflator. In order for a passenger car to be prepared against a side crash, an airbag has recently been provided in the form a side bag or side curtain. An airbag in such a varied form is also stored in a small space such as a side portion of a car sheet or a roof/pillar portion. In order to ensure ample occupant space in a car, without reducing it under such circumstances, it is required to make the stowed volume of the airbag as small as possible and, further, to make the airbag system light.
In order to make the airbag compact and lightweight to meet the requirements, a base fabric for airbag that is a lightweight woven fabric is required.
In order to make the stowed volume of an airbag small, a weaving yarn having a small yarn size has been used as a yarn for the woven fabric. In order to make an airbag airtight, the type and amount of an elastomer to be applied to the base woven fabric have been adjusted. For example, the size of a yarn to be used for the woven fabric is decreased from 940 dtex to 470 dtex. Moreover, the elastomer has been changed from chloroprene to silicone, and the coating amount has been decreased, from 90 to 120 g/m2, to from 40 to 60 g/m2. A coated fabric prepared by coating a base woven fabric having a yarn size of 470 dtex with a silicone resin is currently being used.
Recently, airbags have been required to be even more lightweight and compact. Accordingly, references such as WO 01/09416 disclose the use of a weaving yarn having a smaller yarn size (from 67 to 250 dtex) for the purpose of making a woven fabric for an airbag more lightweight, and the use of a weaving yarn having a smaller size filament (single filament size of from 0.5 to 4.5 dtex) for the purpose of making the airbags compact or making the woven fabric give a soft feel.
On the other hand, in order to make the coated fabric lightweight, it is desired that the coating amount is decreased. However, when the coating amount of silicone is decreased, the burning speed increases, and a problem, that the burning speed exceeds the upper limit that FMVSS 302 regulation specifies, arises. For example, Japanese Unexamined Patent Publication (Kokai) No. 7-300774 discloses the production of a coated fabric that passes the flame proofness test, which comprises coating a fabric with a mixture prepared by adding solid powder of a substance such as acetylene black and Fe2O3 to silicone. However, the coated fabric is not lightweight, because the coating disclosed has a thickness from 5 to 20 μm. Moreover, when powder of a substance such as acetylene black or Fe2O3 is added to a silicone composition in an amount of from 5 to 10 mass parts as a solid component, the solid powder cannot be sufficiently mixed with silicone. As a result, the following problems arise: the operator or the surroundings is contaminated with solid powder that drops off the coating during handling; solid powder that scatters during sewing an airbag clogs the eye of a sewing needle, and the sewing machine must be frequently repaired. When a solid powder accumulates in the eye portion of a sewing needle and/or thread guides, there is a possibility that a critical problem takes place in the step of sewing with an industrial high speed sewing machine due to a variation in the tension of feeding a sewing yarn. A variation in the tension of feeding a sewing yarn destroys the balance between the tension of a needle thread and that of a bobbin thread, resulting in the formation of a grinning seam to vary the thickness of an airbag product. Moreover, the tension of the running yarn is varied to damage the sewing yarn or to produce a yarn break. As a result, a problem, that the reliability of the airbag deployability might be impaired, arises.
Japanese Unexamined Patent Publication (Kokai) No. 2001-138849 describes a lightweight coated fabric with excellent stowability that is prepared from a woven fabric formed from a yarn with a yarn size of from 100 to 250 denier and coated with a silicone rubber in an amount of from 5 to 35 g/m2, and that can prevent a gas leak. However, the patent publication discloses neither a sufficient suppression of burning speed with a light silicone coating nor the design of a coated fabric that is a lightweight woven fabric and that has mechanical properties sufficient for ensuring an inflating pressure resistance of the airbag.
Japanese Unexamined Patent Publication (Kokai) No. 5-319194 discloses an attempt to improve the softness of an airbag by a procedure of applying a silicone to the base woven fabric. In the attempt, an airbag base fabric formed of a weaving yarn having a yarn size of from 420 to 840 denier is impregnated with an amino-modified silicone as a softening agent and a methylhydrogensilicone (0.11 to 0.49% by weight) as a water repellent to form an air permeable woven fabric layer. The woven fabric is further coated with a silicone elastomer to form an air impermeable coating layer (35 to 65 g/m2). The patent publication describes that an impact on a human body caused by the deployment of the airbag can be made small, as a result. However, when a base woven fabric formed from a yarn with a yarn size as small as 270 dtex or less and having a lightweight coating of 25 g/m2 or less is prepared by the above method in order to aim at a lightweight airbag, the coated base woven fabric cannot pass the FMSS 302 burning test.
Japanese Unexamined Patent Publication (Kokai) No. 11-1876 discloses an embodiment in which the pressure of an airbag is maintained by coating a hollow woven base fabric formed of a weaving yarn having a yarn size of 420 denier with a double-layered silicone elastomer. The patent publication describes as follows. The first layer contains a hydrogensilicone that is a molecular chain extender and a silicone with a high breaking point elongation caused by a reinforcing filler of fumed silica. The second layer contains a silicone having a high tear strength and crosslinked with a vinylsilicone having three or more functional groups. The coating in a coating amount of from 60 to 220 g/m2 acts to maintain the pressure. However, the patent publication does not disclose the design of a coated fabric that can maintain the pressure of an airbag while it makes the airbag lightweight.
During the deployment of an airbag, hot particles derived from the burnt residue of a gas-generant in the inflator sometimes form a melt hole, termed a burnt-through-hole, in the bag. When the heat capacity of an airbag fabric that is made from small yarn size synthetic fiber is small, the possibility of forming a melt hole becomes high. Accordingly, means for preventing the bursting of an airbag, that starts from the melt hole, is required. Moreover, ideally, an airbag must deploy immediately at the time of a collision to restrain an occupant. Therefore, an airbag that shows a short deployment time is also required.